Ontogeny of opiate receptors in rat forebrain: visualization by in vitro autoradiography

The embryonic and postnatal ontogeny of opiate receptors in rat telencephalon was mapped by in vitro autoradiographic localization of [3H]naloxone and [3H]enkephalin binding. Opiate receptors marked by naloxone binding first appear at embryonic day 14 in the striatum, rapidly proliferate to adult densities and at the time of birth, gradually become reorganized into the adult heterogeneous pattern of receptor-rich patches surrounded by sparse, diffuse labeling in the rest of the striatum. The enkephalin binding in the striatum appears later in embryonic development and gradually increases in density to form the rather homogeneous adult pattern. Naloxone binding in the paleocortical olfactory areas appears early also, densely within the molecular layer as soon as it is formed at E16. This density is only temporary, as labeling just after birth falls to low adult levels in all areas except portions of the amygdala. Receptors disappear also in the islands of Calleja and the pallidum. Naloxone binding in the septum and neocortex appears gradually in development. The early appearance of striatal and paleocortical [3H]naloxone-labeled opiate receptors and their localization within the subependymal zones suggest that receptors appear on immature neurons before and during migration and, therefore, may influence the intricate patterns of connections that later form. The delayed appearance of the [3H]enkephalin-labeled receptors may reflect the dependence of the peptide binding on later developing molecules of adenylate cyclase.